Numerous semiconductor devices can be used as power switches. These devices include thyristors, bipolar junction transistors, junction field effect transistors, insulated gate field effect transistors, field controlled thyristors, insulated gate controlled thyristors, and bipolar mode conductivity modulated field effect transistors. The choice of which device is preferred for a particular application is a function of many factors. These factors include the desired current handling capability of the device, the forward and reverse blocking voltages desired, turn-on and turn-off time desired, typical and maximum switching frequencies desired, circuit environment considerations such as resistive/inductive loads encountered, drive requirements, cost, etc.
Some applications of power switches require switching of low voltages at high currents. For example, automotive applications require switching of approximately 12 volts at currents of about 10-100 amperes. Using semiconductor devices rather than electromechanical relays to do this switching offers advantages of reliability, cost and ease of use in interfacing with control circuitry. Such advantages are obviously significant.
There are several characteristics which are particularly desirable in thyristor devices used for automotive switching applications. Such characteristics include a grounded cathode for compatibility with the automotive electrical system and control by means of low current and low positive voltages. This provides simpler interfacing and better electrical compatibility with other circuits in the automotive system. In addition, it is desired that the thyristor not only be rapidly turned on, but also rapidly turned off, by the low power positive control voltages. Low cost is very important too. In order to achieve low cost, a high current density device with a low on-resistance is desired. This minimizes silicon chip area in the device and thereby improves yields, while keeping heat sinking and packaging costs to a minimum.
The known type of thyristor having an insulated gate for turn-on has most of the foregoing desirable features. However, it cannot be turned off unless current density drops to a very low value. I have found how to add an insulated gate to such a device that provides rapid turn-off even when anode voltage stays high.